Promise

What

The value in promise is permitted to mutate at most once.

• Created:

  • Pending: Empty box that contains nothing

• Eventually:

  • Resolved: like putting something inside the box.
  • Rejected: box filled with an Exception
  • Contents may never change after resolved or rejected

• The client code that wants to make use of concurrency will need to access promises: resolved or pending, and make use of the resolved values.

• The library and OS code that implements concurrency will need to mutate the promise, to actually resolve or reject it. Client code does not need to mutate the promise.

Abstraction Layer resolver

• Create a new promise and its associated resolver. The promise is pending.
• Call an OS function that will concurrently read the string, then invoke the resolver on that string.
• Return the promise (but not resolver) to the client. The OS meanwhile continues to work on reading the string.